This invention relates to the field of lithography and, more particularly, to an improved base plate adapted for use in the preparation of either presensitized or wipe-on substractive lithographic plates, to an improved sensitized plate which may be prepared from the base plate, and to methods for preparing such plates.
Among the most desirable and widely used light sensitizers for negative-acting lithographic plates are the various diazo resins, in particular those prepared by condensation of a 4-diazo-1,1'-diphenylamine with formaldehyde or other carbonyl compounds. Coated on an appropriate hydrophilic support, such as aluminum, these diazo resins may be exposed and developed to provide sharp ink-receptive serviceable images for lithographic printing.
Diazo resins are conventionally prepared by condensation in an acid medium such as sulfuric or phosphoric acid. The resin is typically precipitated as a double salt, for example with zinc chloride. Depending upon the medium in which the condensation reaction is carried out, the diazo resin may be either organic solvent-soluble or water-soluble. Because of the expense, hazard and pollution problems associated with organic solvents, water-soluble diazo resins are often preferable from a processing standpoint, especially in wipe-on operations and in relatively low volume printing operations where the inconvenience and expense of handling and recovering organic solvents may be a serious burden.
Because diazo resin images tend to be rather fragile, diazo-sensitized lithographic plates are generally not well adapted for strictly subtractive development, nor is the press life exhibited by such plates typically very high. In order to provide an image of the toughness and oleophilicity required for longer run printing, so-called "add-on" developers are commonly used in the preparation of an exposed diazo-sensitized lithographic plate for printing. Add-on developers contain an organic resin component such as an epoxy which forms a durable oleophilic add-on coating over the diazo image. Add-on developers are effective in the preparation of plates for printing but their application requires care and expertise if satisfactory results are to be realized.
On the basis of durability, hydrophilicity and cost, aluminum is the preferred support for lithographic plates. Since diazo resins are reactive with and deteriorate on a bare aluminum surface, however, aluminum was initially considered suitable only as a substrate for freshly prepared diazo-sensitized plates. Subsequently, numerous barrier layers, for example, sodium silicate, have been utilized to allow the use of aluminum as a support for diazo resin plates which may be stored. Barrier layers of various types are also commonly used on other conventional supports such as zinc, magnesium, paper and plastic.
Although barrier layers greatly enhance the storage stability of plates having an aluminum support and a diazo sensitizer, the above-noted fragility of diazo resin images has tended to limit the press life of both wipe-on and presensitized diazo resin plates. In the case of wipe-on plates, the techniques of application, exposure, and development all require some expertise, and inconsistent results may be obtained if the appropriate technique is not carefully and meticulously followed.